Recently, TFTs using an oxide semiconductor layer containing indium (In), zinc (Zn), gallium (Ga) or the like have been actively developed (e.g., Patent Documents 1 through 4). A TFT using an oxide semiconductor has a feature of having a high mobility.
However, in the case of the TFT having an oxide semiconductor layer, in the step of etching for forming source and drain electrodes, the oxide semiconductor layer below the layer to be etched is likely to be damaged. Therefore, there is a problem that in the case where an oxide semiconductor layer is used as a channel region of the TFT, it is difficult to obtain stable TFT characteristics.
As a measure against this, Patent Document 3, for example, proposes forming an insulating layer for protecting a channel region (hereinafter, this insulating layer will be referred to as a “protective layer”) on the oxide semiconductor layer. Such a structure, for example, prevents the resistance of the channel region of the oxide semiconductor layer from being reduced due to oxygen deficiency, which is caused in the step of etching for forming the source and drain electrodes. As a result, TFT characteristics with a small OFF currency can be realized.